Improve error messages for bad arity errors

Author: José Valim

lib/elixir/lib/exception.ex

@@ -54,9 +54,16 @@ end
 
 defexception BadArityError, [function: nil, args: nil] do
   def message(exception) do
-    args = Enum.map_join(exception.args, ", ", &inspect/1)
-    "bad arity error: #{inspect(exception.function)} called with (#{args})"
+    fun  = exception.function
+    args = exception.args
+    insp = Enum.map_join(args, ", ", &inspect/1)
+    { :arity, arity } = :erlang.fun_info(fun, :arity)
+    "#{inspect(fun)} with arity #{arity} called with #{count(length(args), insp)}"
   end
+
+  defp count(0, _insp), do: "no arguments"
+  defp count(1, insp),  do: "1 argument (#{insp})"
+  defp count(x, insp),  do: "#{x} arguments (#{insp})"
 end
 
 defexception UndefinedFunctionError, [module: nil, function: nil, arity: nil] do

lib/elixir/lib/inspect.ex

@@ -481,7 +481,7 @@ defimpl Inspect, for: Function do
   end
 
   defp default_inspect(mod, fun_info) do
-    "#Function<#{uniq(fun_info)} in #{Inspect.Atom.inspect(mod)}.#{extract_name(fun_info[:name])}>"
+    "#Function<#{uniq(fun_info)}/#{fun_info[:arity]} in #{Inspect.Atom.inspect(mod)}.#{extract_name(fun_info[:name])}>"
   end
 
   defp extract_name(name) do

lib/elixir/test/elixir/exception_test.exs

@@ -62,7 +62,7 @@ defmodule Kernel.ExceptionTest do
 
   test "format_fa" do
     assert Exception.format_fa(fn -> end, 1) =~
-           %r"#Function<\d\.\d+ in Kernel\.ExceptionTest\.test format_fa/1>/1"
+           %r"#Function<\d\.\d+/0 in Kernel\.ExceptionTest\.test format_fa/1>/1"
   end
 
   test "runtime error message" do

lib/elixir/test/elixir/kernel/rescue_test.exs

@@ -169,7 +169,7 @@ defmodule Kernel.RescueTest do
 
   test :badarity_error do
     fun    = fn(x) -> x end
-    string = "bad arity error: #{inspect(fun)} called with (1, 2)"
+    string = "#{inspect(fun)} with arity 1 called with 2 arguments (1, 2)"
 
     result = try do
       fun.(1, 2)